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Synergy between the Mos/mitogen-activated protein kinase pathway and loss of p53 function in transformation and
1ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Maryland 21702-1201, USA.
Abstract:
Constitutive activation of mitogen-activated protein kinase (MAPK) is a property common to many oncoproteins, including Mos, Ras, and Raf, and is essential for their transforming activities. We have shown that high levels of expression of the Mos/MAPK pathway in Swiss 3T3 fibroblast cause cells in S phase to undergo apoptosis, while cells in G1 irreversibly growth arrest. Interestingly, cells in G2 and M phases also arrest at a G1-like checkpoint after proceeding through mitosis. These cells fail to undergo cytokinesis and are binucleated. Thus, constitutive overexpression of Mos and MAPK cannot be tolerated, and fibroblasts transformed by Mos express only low levels of the mos oncogene product. Here, we show that p53 plays a key role in preventing oncogene-mediated activation of MAPK. In the absence of p53 (p53-/-), the growth arrest normally observed in wild-type p53 (p53+/+) mouse embryo fibroblasts (MEFs) is markedly reduced. The mos transformation efficiency in p53-/- MEFs is two to three orders of magnitude higher than that in p53+/+ cells, and p53-/- cells tolerate > 10-fold higher levels of both Mos and activated MAPK. Moreover, we show that, like Mos, both v-ras and v-raf oncogene products induce apoptosis in p53+/+ MEFs. These oncogenes also display a high transforming activity in p53-/- MEFs, as does a gain-of-function MAPK kinase mutant (MEK*). Thus, the p53-dependent checkpoint pathway is responsive to oncogene-mediated MAPK activation in inducing irreversible G1 growth arrest and apoptosis. Moreover, we show that the chromosome instability induced by the loss of p53 is greatly enhanced by the constitutive activation of the Mos/MAPK pathway.
Insights
The tumor suppressor p53 prevents oncogene-driven MAPK pathway activation, halting cell growth or causing apoptosis. Loss of p53 enhances oncogene transformation and chromosome instability.
Area of Science:
- Cell Biology
- Molecular Oncology
- Cancer Genetics
Background:
- Constitutive activation of the mitogen-activated protein kinase (MAPK) pathway by oncoproteins like Mos, Ras, and Raf is crucial for cell transformation.
- Overexpression of the Mos/MAPK pathway in fibroblasts induces apoptosis in S-phase cells and irreversible G1 growth arrest in others, with G2/M phase cells arresting post-mitosis, failing cytokinesis, and becoming binucleated.
- Fibroblasts transformed by Mos typically express low levels of the oncogene product, suggesting a tolerance limit for pathway activation.
Purpose of the Study:
- To investigate the role of p53 in preventing oncogene-mediated MAPK pathway activation and its impact on cell cycle control and transformation.
- To determine how the absence of p53 affects the cellular response to oncogenes that activate the MAPK pathway.
- To elucidate the interplay between p53, MAPK signaling, and chromosome instability in the context of oncogenic transformation.
Main Methods:
- Utilizing mouse embryo fibroblasts (MEFs) from wild-type (p53+/+) and p53-deficient (p53-/-) mice.
- Assessing the impact of Mos, v-Ras, v-Raf, and a constitutively active MAPK kinase mutant (MEK*) on cell growth, apoptosis, and transformation efficiency in both p53+/+ and p53-/- MEFs.
- Quantifying levels of Mos and activated MAPK in different cellular contexts.
- Evaluating chromosome instability in response to combined p53 loss and MAPK pathway activation.
Main Results:
- The absence of p53 significantly reduces the growth arrest response to oncogene-mediated MAPK activation, leading to a 2-3 log increase in Mos transformation efficiency in p53-/- MEFs compared to p53+/+ cells.
- p53-/- cells tolerate over 10-fold higher levels of Mos and activated MAPK compared to their wild-type counterparts.
- Oncogenes Mos, v-Ras, and v-Raf, as well as a gain-of-function MEK mutant, exhibit higher transforming activity in p53-/- MEFs than in p53+/+ MEFs, inducing apoptosis in the latter.
- The p53-dependent checkpoint pathway effectively responds to oncogene-mediated MAPK activation by inducing G1 growth arrest and apoptosis.
- Loss of p53 greatly enhances chromosome instability when combined with constitutive activation of the Mos/MAPK pathway.
Conclusions:
- p53 plays a critical role as a tumor suppressor by enforcing cell cycle checkpoints (G1 arrest and apoptosis) in response to oncogenic MAPK pathway activation.
- The lack of p53 function allows cells to tolerate higher levels of MAPK signaling and promotes oncogene-induced transformation and genomic instability.
- These findings highlight the importance of the p53 pathway in safeguarding against the detrimental effects of aberrant MAPK signaling during oncogenesis.